1. Technical Field of the Invention
This invention relates generally to multi-zone forced-air HVAC systems, and specifically to control methods for reducing conditioning and energy consumption.
2. Background Art
Most zone control systems for residential forced-air HVAC systems have a small number of zones in combination with HVAC equipment that has fixed capacity or variable capacity over a limited range or discrete steps of capacity. Simple zone control systems have a convention thermostat for each zone. Each zone has and airflow control damper that is opened or closed by signals from the thermostat for that zone. The calls for conditioning from each thermostat are combined using a logical OR function. The conditioning equipment runs when one or more thermostats make a call for conditioning. When a thermostat calls for conditioning, the damper for that zone is open. When the zone thermostat is not calling for conditioning, the damper for that zone is closed. Each zone operates independently without knowledge of the conditioning of the other zones.
One problem with simple zone control is that the amount of conditioned airflow needed depends on the number of zones calling for conditioning. For example, in a system with four equal zones, each zone might be capable of receiving only 25% of the total capacity. If the HVAC equipment has fixed capacity and only one zone calls for conditioning, 75% of the airflow is excess capacity. Various strategies are used in the prior art for dealing with this excess airflow capacity.
A simple strategy is to oversize the duct work to each zone so it can receive 100% of the airflow produced by the HVAC equipment. However, the extra ducting is expensive to install and requires space that may not be available. This is usually not practical for retrofit. In addition, when multiple zones call for conditioning, the airflow velocity to each zone is reduced, so the conditioned air may not mix properly with the unconditioned air in the zones. This may produce warm and cool areas within the zones.
Another strategy for managing the excess airflow is to use a controllable bypass duct to shunt supply airflow directly to the return airflow. The bypass typically opens automatically as the supply pressure increases, providing a path for some of the excess conditioned airflow. U.S. Pat. No. 5,249,596 issued Oct. 5, 1993 to Hickenlooper, III et al. describes a bypass damper for use in such zone control systems.
A significant problem with using a bypass is the return air becomes heated or cooled. When in heating mode, excessive bypass airflow can heat the return air temperature above 85°. This exceeds the recommend operating conditions for most residential HVAC equipment, voiding the manufacturer's warranty. When in cooling mode, excessive bypass can reduce the return air temperature sufficiently to freeze the evaporator coil. To prevent excessive return air temperatures in most HVAC systems, the maximum bypass airflow must be less than about 20% of the total conditioned airflow.
Another problem with using a bypass is that it shifts the effective operating temperature of the heat exchange process. This usually reduces the energy efficiency of the equipment and can reduce equipment lifetime.
Another strategy for dealing with excess conditioned airflow is to only partially close the dampers of at least some of the zones that are not calling for conditioning. In some systems, the dampers have mechanical stops that must be set and adjusted during the installation process or in a follow up service call. In other system, the damper positions are set dynamically by a control process. U.S. Pat. No. 5,829,674 issued Nov. 3, 1998 to Vanostrand, et al. describes a multi-zone control system that uses modulating dampers. These control systems are designed primarily for temperature balancing between zones to maximize comfort. The control methods are not optimized for energy savings.
Another strategy for dealing will excess conditioned airflow is to use HVAC equipment that has variable capacity. In these systems, the total needs of all the zones are considered when setting the output capacity of the HVAC equipment. Some variable capacity HVAC equipment provides two discrete stages where the first-stage produces 60% to 70% of the conditioned airflow as the second-stage. Other equipment can be adjusted continuously from about 30% to 100% based on the required airflow for the zones that require conditioning. U.S. Pat. No. 5,863,246 issued Jan. 26, 1999 to Bujak, Jr. describes a zone control system where the conditioning capacity of the HVAC equipment is adjusted to match the needs of the zones calling for conditioning.
Any zone system should improve the temperature control in a building. More zones provide better temperature control. Zone systems can potentially reduce the energy used to condition a building, but the energy savings depends on the details for the building, the zone system, and how the occupants set the zone temperatures. Some zone systems actually use more energy because the excess airflow is inefficiently managed.
Zone systems can save energy by selectively conditioning areas based on occupancy and activity. Areas that are occupied are conditioned only as much as needed, and areas that are unoccupied are conditioned as little as possible. Energy savings depends on the zone areas matching occupancy areas and the ability of occupants to easily set temperatures that match their occupancy patterns. In addition, settings that save energy when an area is unoccupied should not affect the comfort of that area when occupied.
In a typical zone control system for use in single family homes, a zone includes several rooms. The airflows to all rooms in the zone are controlled by one thermostat. To provide good temperature control, all rooms in the zone must have good thermal coupling with the zone thermostat. Zones must be related to the geometry of the home rather than the use of the rooms in the zone. For example, a two-zone system typically divides a home into a living area and a sleeping area or an upstairs area and a downstairs area. Using different temperature settings for each zone for different times of the day can reduce the energy used for conditioning. However, the actual occupancy pattern may not match the zone organization. For example, one bedroom might be used as a home office. Or one bedroom may be a nursery occupied full time by an infant. School children may use their bedroom in the afternoon for homework or play, or use it all day in the summer. If one room in the zone is occupied, then the entire zone must be conditioned for occupancy. Likewise one person may use one room of the living space early in the morning and a different person use another room in the living space late at night. With only two zones, it is likely that at least one room in each zone is occupied most of the time. There is little opportunity to reduce the conditioning to save energy.
The best opportunity for energy savings while maximizing comfort is to make every room a separate zone, providing a temperature sensor, temperature settings, and airflow control for every area that has a supply vent and a door or different thermal environment. An average 2500 square ft home has 10-15 separate rooms and areas with different thermal environments, so a 10-15 zone system should be used. Such a multi-zone control system for residential use is disclosed in U.S. Pat. No. 6,786,473 issued Sep. 7, 2004 to Alles, U.S. Pat. No. 6,893,889 issued Jan. 10, 2004 to Alles, U.S. Pat. No. 6,997,390 issued Feb. 14, 2006 to Alles, U.S. Pat. No. 7,062,830 issued Jun. 20, 2006 to Alles, U.S. Pat. No. 7,162,884 issued Jan. 16, 2007 to Alles, U.S. Pat. No. 7,188,779 issued Mar. 13, 2007 to Alles, and U.S. Pat. No. 7,392,661 issued Jul. 1, 2008 to Alles. These patents describe various aspects of a HVAC zone control system that uses inflatable bladders and various control methods. This system is designed for retrofit and to use the existing HVAC systems in residential single family homes. Homes larger than 2500 sq ft typically have 12-30 vents, each with an airflow capacity only a small fraction of that supplied by the HVAC equipment. Therefore any time the HVAC equipment is run, a minimum number of vents must be open to provide sufficient airflow capacity to allow the HVAC equipment to operate efficiently. Even if a single room calls for conditioning, the HVAC equipment should be run to provide comfort in that room. This means that several rooms that are not calling for conditioning must also be conditioned.
U.S. Pat. No. 7,188,779 issued Mar. 13, 2007 to Alles describes a method for selecting zones to receive a portion of the excess conditioning from among those zones not calling for conditioning. Non-calling zones are incrementally selected for conditioning until total airflow capacity is sufficient to receive the airflow generated by the HVAC equipment. The priority for selecting non-calling zones is primarily based on the zone's nearness to needing conditioning. In the simplest terms, this is determined by the difference between the zone's temperature and its set point. The unconditioned and non-calling zone with its temperature closest to its set point is the next zone selected for conditioning.
This method produces good results for comfort, but may use more energy for conditioning than necessary when many zones are unoccupied. If many zones are set for minimum conditioning because they are unoccupied, the excess conditioned air tends to be distributed to all of the non-calling zones such that their temperatures are all about the same. In most cases, energy can be saved by conditioning only a specific subset of the non-calling zones while providing little or no conditioning to other non-calling zones. As a result, the temperature difference between some non-calling zones can be quite large. However, less total conditioning, and therefore less energy is needed to condition the occupied zones to their set temperatures.